1. Field of the Invention
The present invention relates to a method for producing a nanostructure, and, more particularly, to a method for producing a nanostructure with narrow pores, which can be utilized in a wide range of applications to functional materials, structural materials, etc., for electron devices, optical devices, microdevices or the like, and the nanostructure.
2. Related Background Art
Thin films, narrow wires, dots, and so on of metals and semiconductors, when constructed in the size smaller than a certain characteristic length, sometimes demonstrate peculiar, electrical, optical, and/or chemical properties because of confinement of electrons. From this viewpoint, attention is being drawn to materials having the structure finer than several hundred nm (nanostructures) as functional materials.
Methods for producing such nanostructures are, for example, methods for directly producing the nanostructures by semiconductor producing techniques including the fine pattern forming technologies such as the photolithography, electron beam exposure, X-ray exposure, and so on.
In addition to the above production methods, there are attempts to realize novel nanostructures on the basis of ordered structures formed naturally, i.e., structures formed in a self-ordering manner. Many studies have been started on these techniques because they have the potential capability of producing finer structures than by the conventional methods, depending upon the microstructure used as a basis.
An example of such self-ordering techniques is anodic oxidation which permits the nanostructure with narrow pores of the nanometer size to be produced readily and with good controllability. A known example is anodized alumina produced by anodizing aluminum and an alloy thereof in an acid bath.
When an Al sheet is anodized in an acid electrolyte, a porous oxide film is formed (for example, see R. C. Furneaux, W. R. Rigby & A. P. Davids, "NATURE," Vol. 337, P147 (1989) etc.). A feature of this porous oxide film is the peculiar, geometrical structure in which extremely fine, cylindrical holes (nanoholes) having the diameters of several nm to several hundred nm are arranged in parallel at the spacing (cell size) of several nm to several hundred nm. These cylindrical holes have high aspect ratios and are also excellent in uniformity of sectional sizes. The diameter and spacing of the narrow pores can be controlled to some extent by adjusting current and/or voltage during the anodic oxidation, and the thickness of the oxide film and the depth of the narrow pores can be controlled by controlling the time of the anodic oxidation.
In order to improve vertical, linear, and independence characteristics of the narrow pores, there was a proposal of a method of carrying out the anodic oxidation in two stages, i.e., methods of producing the narrow pores by once removing the porous oxide film formed by the anodic oxidation and thereafter carrying out the anodic oxidation again ("Jpn. Journal of Applied Physics," Vol. 35, Part 2, No. 1B, pp.L126-L129, issued Jan. 15, 1996). This method makes use of the fact that pits formed in the surface of the aluminum sheet after the anodized film by the first anodic oxidation is removed, serve as starting points for formation of narrow pores by the second anodic oxidation.
Further, in order to improve the shape and spacing of the narrow pores and the controllability of the pattern, there was also a proposal of a method of forming the fine-hole-formation starting points by use of a stamper, i.e., a method of producing the narrow pores with better shape, spacing, and controllability of pattern by forming depressions as fine-hole-formation starting points while urging a substrate with a plurality of projections in the surface against the surface of the aluminum sheet, and thereafter carrying out the anodic oxidation (Japanese Patent Application Laid-Open No. 10-121292).
There have been attempts of various applications, noting this peculiar, geometrical structure of anodized alumina. Masuda explains them in detail. Application examples thereof will be described below. For example, there are applications to films making use of wear resistance and electric insulation of the anodized films and applications to filters using peeled off films. Further, there were attempts of various applications including coloring, magnetic recording media, EL light-emitting devices, electrochromic devices, optical devices, solar cells, gas sensors, and so on, by using the technology of filling the nanoholes with metal or a semiconductor or the like and the replica technology of nanoholes. In addition, the technologies are expected to be applied in many fields including quantum effect devices such as quantum wires, MIM devices, and the like, molecular sensors using the nanoholes as chemical reaction fields, and so on (Masuda "KOTAIBUTSURI" (Solid State Physics), 31, 493 (1996)).